1. Field of the Invention
The present invention relates to signal correlators for digital signal receivers and, more particularly, the invention relates to a method and apparatus for performing signal correlation for signals received from satellites in multiple satellite systems, for example, a global positioning system (GPS) satellite and a Galileo satellite.
2. Description of the Background Art
A Global Satellite Navigation System (GNSS) has been contemplated to utilize satellite signals from both Global Positioning System (GPS) satellites and Galileo System satellites to facilitate position location using signals from satellites in either system. However, satellite signal receivers are generally designed to receive signals from satellites in one satellite system, e.g., the Global Positioning System or Galileo, but not both. As such, the receiver must wait for at least four GPS satellites to be visible and provide strong enough signals to enable the satellite signal receiver to acquire the signals. The constellation of 24 GPS satellites is generally sufficient for users to determine there position in an open area, i.e., 4 to 8 satellites will generally be visible to the receiver. However, when a GPS receiver is operating in an area where trees, mountains or buildings occlude the view of the sky, the receiver may not determine an accurate position, or may not function at all.
To enable a GNSS receiver to receive satellite signals at low signal levels, assistance information is sent to the receiver to enable the receiver to acquire and track the signals. Such assistance information may include satellite orbit information, timing synchronization information, Doppler frequency information, and the like. A GNSS receiver generally contains a correlation circuit that locates and tracks a correlation peak of a received satellite signal. The assistance information is used to assist the correlator circuitry in identifying an appropriate code phase and frequency estimate to use to acquire and track the satellite signal.
Since each GNSS system uses a different modulation technique, the correlator circuits differ between a Galileo receiver and a GPS receiver. For example, a GPS satellite transmits a pseudorandom-noise (PRN) modulated signal that uses a repeating 1023 chip code in a 1 mS epoch, while Galileo uses a 4092 chip code in a 4 mS epoch, but also adds a Binary Offset Carrier (BOC) code that “spreads” the signal to twice the bandwidth of a GPS signal, i.e., the 4092 bit PRN code is modulated with a binary signal at twice the rate of the PRN code.
Due to these signal differences, receivers of GPS and Galileo signals are not compatible. As such, to produce an accurate position, a receiver designed for a single satellite system must have sufficient signal strength from four satellites within that satellite system. In certain situations, this is not possible and a receiver will fail to compute a position or will require a very long time to compute a position. Alternatively, the receiver would have correlator circuitry dedicated to each satellite system; thus approximately doubling the size of the circuitry within the receiver.
Thus, there is a need for an improved, satellite signal receiver that acquires signals from satellites in multiple satellite systems using common correlator circuits.